Ultrasonic flow rate meters have been constructed which measure a flow velocity of a liquid or gas based on time-of-flight measurements. Typically, a pair of ultrasonic transducers is arranged at opposite ends of a flow tube having a known length. Alternatively, a pair of ultrasonic transducers may be arranged spaced apart along the length of a flow tube by a distance, the ultrasonic transducers arranged at angles to the flow tube such that ultrasound may pass between the pair by reflecting from an internal wall or reflector of the flow tube. By obtaining time-of-flight measurements between the ultrasonic transducers both with, and against, the flow direction of the liquid or gas, a difference may be obtained which relates to the flow velocity of the liquid or gas.
For example, US 2013/080,081 A1 describes a fluid flow meter which estimates the velocity of water or another fluid flowing through a pipe by comparing measurements of the water velocity to one or more pre-determined templates. The fluid flow meter may collect measurement signals from one or more flow sensors (e.g., ultrasonic transducers), estimate the fluid velocity or flow rate by comparing the measurement signals to the template(s), and either store the comparison results in local memory, transmit the results to a remote memory or server, or both.
The use of variable frequency, or “chirp”, excitation signals for ultrasonic transducers has been reported, see for example “Excitation signal's influence on ultrasonic transit time flow meter's performance”, L Svilainis, P Kabisius, A Aleksandrovas and A Chaziachmetovas, TOP Conference Series, Materials Science and Engineering, Volume 42, conference 1, DOI: 10.1088/1757-899X/42/1/012047.